The invention relates to stabilized solutions of radiation-crosslinkable polymer precursors of highly heat-resistant polymers.
The polymer precursors involved here are soluble polyaddition or polycondensation products formed from carbocyclic or heterocyclic compounds having two functional groups capable of polyaddition or polycondensation reactions and having radiation-reactive radicals in part attached in the ortho-position or pera-position relative to these groups by an ester linkage to carboxyl groups, and diamines, diisocyanates, bis-acid chlorides or dicarboxylic acids which are capable of reacting with these functional groups and which contain at least one cyclic structural element. The multifarious radiation-reactive radicals which can be present in these soluble polymeric precursors are disclosed, for example, in the following patent specifications: German Patent Specification No. 2,308,830/U.S. Pat. No. 3,975,512, German patent specification No. 2,437,348/U.S. Pat. No. Re. 39,186, German patent specification No. 2,437,368/U.S. Pat. No. 4,045,223 and German patent specification No. 2,437,422/U.S. Pat. No. 4,088,489 and in German patent applications, German Offenlegungsschriften Nos. 3,227,584 and 3,233,912, equivalent to U.S. Pat. No. 4,540,650, all of which disclosures are incorporated by reference herein.
As a rule, these are radiation-crosslinkable unsaturated groups containing vinyl or allyl double bonds. In particular, they are hydroxyallyl, hydroxyalkyl acrylate and hydroxyalkyl methacrylate groups which are attached by an ester linkage to carboxyl groups, such as, for instance, 2-hydroxyethyl acrylate or methacrylate groups, and also ester groups containing allyloxy and/or allylthio groups.
These radiation-crosslinkable polymer precursors which are readily soluble in a large number of organic solvents can be processed very advantageously to give photosensitive resist lacquers which are used to a large extent in the production of electrical and electronic components, for instance as photoresists or in the form of insulation or protective layers in the manufacture of semiconductors, or for the production of orientation layers in the manufacture of liquid-crystal displays or in the production of etch resists and plating resists and of printing plates. This is effected by applying photosensitive resist lacquers of this type to appropriate substrates in the form of a layer, drying the layer and then exposing it imagewise completely or through a negative original. As the result of this, a crosslinking takes place at the irradiated areas, which drastically reduces the solubility of the applied material at these areas. Non-irradiated parts of the layer can then be detached from the substrate by means of a developer; the remaining parts of the layer or relief structures can be converted by heat treatment into highly heat-resistant polymers which withstand temperatures of 250.degree.-400.degree. C. without a disadvantageous effect on the coating or the sharpness of contours and resolution of the image pattern.
It is known, however, that solutions of these polymer precursors, such as can be present for the above-mentioned purposes, for instance in the form of photosensitive resist lacquers, have a tendency towards spontaneous, non-reproducible and also irreversible increases in viscosity. These solutions frequently gel in the course of only a few days, sometimes only in the course of several weeks, and are then no longer suitable for further use. By virtue of this spontaneous, unpredictable and uncontrolled behavior, photosensitive resist lacquers based on these polymer precursors are scarcely capable of storage and use or, if so, only a conditional basis. It can be assumed that these viscosity changes are brought about by spontaneous, non-photoinduced crosslinking of the unsaturated, radiation-reactive groups of the polymer precursors.
It has been found that the polymerization inhibitors with which those skilled in the art are familiar for stabilizing acrylic, vinyl and allyl compounds, such as, for example, hydroquinone or, for instance, heavy metals such as copper, effect an inadequate stabilization, or none at all, in the case of the present radiation-crosslinkable polymer precursors, or are even completely unsuitable for use in photosensitive resist lacquers for the manufacture of semiconductors.